1. Field of the Invention
This invention relates to delayed coking, and more particularly to a delayed coking process in which overhead vapors from a coke drum are passed to a coker fractionator where the coker overheads are separated into a vapor stream, intermediate liquid streams, and a bottom flash zone gas oil stream.
2. Background Art
A coking process of the type referred to above is described in detail in U.S. Pat. No. 4,518,487 to Graf et al. As described in that patent, the product yield distribution from the coker is enhanced by removing a flash zone gas oil stream from the bottom of the coker fractionator rather than returning the stream to the coke drum as coker recycle as was done in earlier coking processes, all as described in detail in the aforementioned U.S. Pat. No. 4,518,487.
While the process described in the "487" patent provides significant improvements, it is subject to the disadvantage of producing a flash zone gas oil stream that is difficult to upgrade for further processing. The stream contains significant amounts of finely divided particulate solids as well as heavy viscous mesophase material. The mesophase material is essentially liquid coke which is entrained in the vapors leaving the coke drum. In order to enhance the value of the flash zone gas oil stream, it needs to be hydrotreated. However, the entrained solids and mesophase material rapidly plug and foul the catalyst bed of a hydrotreater when it is attempted to pass the stream through a hydrotreater. The unhydrotreated flash zone gas oil can be processed in a fluidized bed catalytic cracking unit (FCC unit), but the yield distribution of the unhydrotreated stream is poor due to its high aromatic content and other factors. Prior attempts to filter the flash zone gas oil stream so that it could be hydrotreated have been unsuccessful due to rapid filter plugging, difficulty in regenerating the filter medium, and other factors.